1. Field of the Invention
The present disclosure relates to a spark plug used for an internal combustion engine and the like.
2. Description of the Related Art
A spark plug used for an internal combustion engine and the like has a cylindrical insulator having an axial hole, a center electrode inserted in the front end side of the axial hole, a terminal electrode inserted in the rear end side of the axial hole, and a cylindrical metallic shell provided to the outer circumference of the insulator. Further, the rear end of the terminal electrode inserted in the axial hole is exposed from the rear end of the insulator. Further, the front end of the terminal electrode is fixed to the insulator by a glass seal layer containing a conductive material and glass. In addition, a resistor for suppressing an electromagnetic wave noise that may occur in response to the operation of the internal combustion engine and the like is often provided between the center electrode and the terminal electrode inside the axial hole (see, for example, JP-A-2006-66086).
Further, in general, the glass seal layer and the resistor are formed as follows. A glass powder mixture, which is the component of the glass seal layer, containing glass powder and a conductive material is prepared in advance. Furthermore, a resistor composition, which is the component of the resistor, containing glass powder, a conductive material (such as a carbon black), and ceramic particles is prepared in advance. Then, after the center electrode is inserted in the front end side of the axial hole, the glass powder mixture and the resistor composition are filled up in the axial hole. At this time, the resistor composition and then the glass powder mixture are filled in this order. Next, the front end of the terminal electrode is inserted in the axial hole, and the terminal electrode is pressed toward the center electrode side in a heat. Thereby, the glass powder mixture is heated being directly compressed by the terminal electrode, and the resistor composition is heated being compressed via the glass powder mixture. Then, the glass powder mixture and the resistor composition are cooled and thus solidified. Thereby, the glass seal layer and the resistor are formed.
By the way, the insufficient compression of the glass powder mixture may cause a number of pores to be formed within the glass seal layer. In such a case, the density of the glass seal layer may decrease, which results in a smaller contact area per unit area between the terminal electrode and the glass seal layer. The smaller contact area causes the reduction in the fixing strength of the terminal electrode to the insulator via the glass seal layer. As a result, when an impact is applied to the terminal electrode and the insulator, the terminal electrode may easily fall out from the insulator. Further, at the impact, a gap is easily formed between the terminal electrode and the glass seal layer, which may cause a sudden increase of the resistance between the terminal electrode and the center electrode.
Further, the insufficient compression of the glass powder mixture may cause insufficient compression of the resistor composition to be compressed via the glass powder mixture. In such a case, the formation of a number of pores may cause a reduction in the density of the resistor. The resistor with a smaller density causes fewer conductive paths formed by the conductive material from the rear end side (the terminal electrode side) to the front end side (the center electrode side). Therefore, a part of the conductive paths is oxidized according to the usage, so that the resistance of the conductive paths may shapely increase. As a result, the load life property of the spark plug is degenerated.
In order to overcome the above-described disadvantages, the art in which a recess part is provided at the front end of the terminal electrode has been proposed (see, for example, JP-A-9-63745). According to this art, at the compression of the glass powder mixture, the glass powder mixture is less likely to move around to the outer circumference of the terminal electrode due to the presence of the recess part. This allows a larger pressure to be applied to the glass powder mixture from the terminal electrode. As a result, the glass powder mixture and the resistor composition can be sufficiently compressed, which allows for higher densities of the glass seal layer and the resistor.